The present invention relates to a radiation detector adapted for computed tomography (hereinafter referred to as CT).
Radiation CT apparatuses, e.g., X-ray CT apparatuses of the third or fourth generation, are provided with an X-ray detector which comprises a plurality of X-ray detecting elements arranged in a high-density, one-dimensional configuration. As an example of X-ray detectors of this type, solid state X-ray detectors have recently been used frequently which employ a combination of scintillators and photodiodes in place of the conventional gas ionization chamber. In these solid state X-ray detectors, the scintillators used as X-ray detecting elements can be arranged with high density and at narrow pitches, thus permitting the production of high-resolution images.
FIG. 1 shows an X-ray detecting element array 10 (multichannel scintillator element assembly) of one such prior art X-ray detector, FIG. 2 a multichannel photodiode 15, and FIG. 3 an X-ray detector 19 assembled from the combination of the X-ray detecting element array 10 and the photodiode 15. The X-ray detecting element array 10 includes a plurality of scintillator elements 11 arranged with collimator plates 12 between and outside the same. The scintillator elements 11 and the collimator plates 12 are fixed by bonding. Usually, the collimator plates 12 used are sheets of heavy metal having a high X-ray absorption factor, such as lead or tungsten. These metal sheets are coated on both sides with a light reflecting material which can very efficiently reflect light emitted from the scintillator elements 11.
The multichannel photodiode 14 includes an insulating substrate 18, a semiconductor substrate 15 thereon, and a plurality of photodiode elements 16 as semiconductor light detecting elements arranged at the same pitches as the scintillator elements 11. Each photodiode element 16 is formed with a terminal 17 for signal output. The terminals 17 and terminals of the printed-wire assembly on the insulating substrate 18 are electrically connected by bonding the wires (not shown).
As shown in FIG. 3, the X-ray detecting element array 10 and the multichannel photodiode 14 are joined and fixed together by means of a transparent adhesive agent such as a glass adhesive so that the scintillator elements 11 individually correspond to the photodiode elements 16 in position.
The X-ray detector 19, constructed in this manner, is required to have the following properties.
(1) High X-ray absorption factor. PA1 (2) High efficiency in conversion of absorbed X-rays into light. PA1 (3) High light transmission factor. PA1 (4) Fixed quantity of emitted light independent of temperature. PA1 (5) Limited afterglow and its rapid attenuation. PA1 (6) Emission spectrum wavelength within the range for satisfactory sensitivity of the semiconductor light detecting elements.
Scintillator materials having these properties include Gd.sub.2 O.sub.2 S;Pr phosphor. This phosphor, whose luminous efficiency depends very little on temperature, may suitably be used as a scintillator for a detector of an X-ray CT apparatus, providing satisfactory images without irregular variations in magnitude of signals between channels despite the change of ambient temperature.
It is difficult, however, to grow a single crystal of the phosphor material, and it is impossible to obtain a truly satisfactory one. Practically, therefore, Gd.sub.2 O.sub.2 S;Pr scintillators are necessarily formed by sintering powdered material by the hot isostatic pressing (hereinafter referred to as HIP) method. These scintillators manufactured by such sintering, however, are low in light transmission factor for the following reasons.
(1) The scintillator material itself absorbs light.
(2I) Light is scattered by voids in the sintered compact.
(3) The products are colored by impurities.
Moreover, X-ray CT apparatuses incorporating an X-ray detector using these low-transmission scintillators may be subject to the following drawbacks.
(1) Low X-ray/light detection sensitivity and low signal-to-noise ratio.
(2) Since few signals, among other detection signals, include low-energy components based on incident X-rays, the space resolution of images at low-contrast regions is poor.
Although having some advantageous properties such as high X-ray absorption factor and reduced temperature coefficient of luminous efficiency, the Gd.sub.2 O.sub.2 S;Pr scintillator elements are low in light transmission factor and are not very practical for a radiation detector of X-ray CT apparatuses, owing to the inevitable use of the sintering process.